Veering vs. Backing winds

[Andy Wehrle]

On many occasions I have heard these terms used to describe potential positive or negative factors for a tornadic situation. Now that I understand what each means, am I correct in assuming that, in general, veering winds with height and backing winds at the surface would be the preferable scenario?

 

[Jeff Snyder]

Veering and backing are TRENDS which must be defined either at constant height (or quasi-horizontal surface like a pressure surface, e.g. 850mb) or at a constant location (2D location...e.g. a city) and constant time. In other words, we can either vary time or vary height. Let's say we're going to discuss this in terms of a quasi-horizontal surface (e.g. the surface/ground), which means that we are varying time. Backing winds, then, refer to winds, at a particular location (say, Oklahoma City), which are changing in a counterclockwise direction. For example, winds at 11am are from the SSW, but at 1pm they are from the SSE.

We can also talk about veering and backing winds in terms of a particular/constant time and location but varying height. In this case, backing winds mean that the wind direction is changing in a counterclockwise direction WITH HEIGHT. To help avoid confusion, we typically say that winds are "veering with height" or "backing with height". This is the veering or backing wind profile (profile indicating constant time, varying height) that you hear about.

So, in summary:
+ Veering/backing winds (w/o reference to "with height" or "profile") usually refers to winds that are changing IN TIME at a fixed location and height.
+ Veering/backing of winds WITH HEIGHT implies winds that change in a clockwise/counterclockwise manner at increasing heights at a fixed time and ground location.

As a reminder, veering winds with height implies warm-air advection, while backing winds with height (or a backing wind profile) implies cold air advection. Typically, in the US, we want to see a veering low-level wind PROFILE (so, veering with height) with a backing surface wind tendency. It's also pretty common to see backing winds with height in the mid and upper-levels, which can be favorable since it implies cold-air advection in the mid and upper levels, which can increase instability.

 

[Aaron Kennedy]

Yep. Veering winds refers to those which turn clockwise with height and is representative of warm air advection. Backing winds turn CCW with height and are associated with CAA. Traditionally, winds that turn more easterly at the surface are said to "back". A bit of a misnomer considering this leads to more veering with height.

The big reason that these conditions are favorable for supercells is it can often lead to cyclonically curved hodographs which favor right split storms. In addition, winds "backing" at the surface often leads to streamwise vorticity when storms deviate to the right. Streamwise vorticity is simply the vortex lines aligned with the flow.... sort of like a rotating barbel pole.

EDIT: Jeff beat me... pretty much echoes my sentiments.

Aaron

 

[mikegeukes]

VEERING AND BACKING WIND DEFINED
http://www.theweatherprediction.com/habyhints/48/

RECOGNIZING VEERING AND BACKING WIND ON RADIAL VELOCITY
http://www.theweatherprediction.com/habyhints2/393/

Mike

 

[Skip Talbot]

Jeff, that is the best description of backing and veering that I have read. The difference between veering/backing with height vs with time has always confused me. I've read in glossaries on these terms that both are favorable for tornado development, which didn't make sense seeing that they are basically opposites (unless any directionaly shear is favorable). Thanks for clearing this one up.